Rosetta - a spacecraft in search of pristine matter
Posted: June 23, 2001

In January 2003 the European spacecraft Rosetta is to set off in search of pristine matter. It will be launched on its long flight to the comet "Wirtanen" by the European launch vehicle Ariane 5. For approximately one year, it will orbit this tailed star at a distance of one kilometer and explore it in detail. At the same time, a probe will land on the comet's surface for surface-science investigations and analysis.

An artist's concept of the Rosetta comet probe. Photo: DSS
At Astrium, which is the industrial primecontractor, extensive activities are underway for this challenging project of the European Space Agency ESA. Work on the flight model are in full swing since the beginning of this year. All experiments and the lander shall be completed by early July. The satellite will be integrated and tested until the end of September; the so-called environmental tests will then follow in ESA's Technical Center Estec in Noordwijk/The Netherlands. This inspection phase will last until April of next year. The overall volume of this contract, which will run until mid-2002, totals about half a billion German marks.

Rosetta Tacking through the Planetary System
Rosetta is the third of four major Planetary Cornerstone missions of the European Space Agency ESA. In terms of engineering, it ranks among the most difficult projects that are feasible today. It will further expand Europe's commanding position in cometary research which it had attained with the fly-by of Halley's Comet by the space probe Giotto in 1986.

The flight path itself already calls for new technical solutions. Even the thrust of the powerful Ariane 5 is not sufficient to directly inject the spacecraft on its way to the comet. The probe must rather gather momentum three times in the gravity fields of Mars and the Earth in order to get onto the right track. According to current planning this will take place in 2005 at Mars and the Earth and once again at the Earth in 2008. Only after the third swing-by maneuver will Rosetta be catapulted into the outer regions of the planetary system, where it will encounter comet Wirtanen.

In between these events, Rosetta will be largely on its own. This is the reason why the technically most complex workshare for industry is the so-called avionics pack which is also delivered by Astrium. It contains the software for the onboard computers and the attitude control system. Mind and body must operate faultlessly and to a large degree autonomously. The biggest single item under this contract, however, is instrument platform, which is also under the responsibility of Astrium.

The Asteroids Otawara and Siwa
Before its encounter with the comet, however, Rosetta will pass close to two asteroids. In 2006, it will pass Otawara at a distance of 750 kilometers, and in 2008, after a second fly-by of the Earth, it will pass Siwa at a distance of 1,600 kilometers. Asteroids are bits of rock which orbit the Sun primarily in the area between Mars and Jupiter. They are probably the building blocks for a planet which could not be formed on account of Jupiter's enormous gravitational forces. Therefore, these bodies are also called planetoids or minor planets. Astronomers know very little about them and expect to obtain valuable information on their composition and creation from the data delivered by Rosetta.

Comet Wirtanen
The rendezvous maneuver with comet Wirtanen will be initiated in the spring of 2011 and, in the spring of 2012, that is more than nine years after launch, the spacecraft will have reached its destination. And only then will the most thrilling part of the venture start. At that time Wirtanen will be at a distance of nearly 600 million kilometers to the Earth and 750 million kilometers to the Sun. Here, in the icy cold of deep space, the comet is still inactive, which means it does not emanate any gases which could form a coma or tail.

During that time the surface will be imaged from a distance of approximately one kilometer. For the first time scientists, and of course the public, will be able to clearly see a cometary surface. Details down to one meter are expected to be discernible. The camera will be built by the Max-Planck Institute for Aeronomy in Katlenburg/Lindau, the experts that also developed the successful camera for Giotto.

Simultaneously, spectrometers will scan the surface in various spectral ranges down to the infrared range. This data will be used to determine the mineral and chemical composition of the surface material. The surface is expected to have the appearance of a dirty, crusted icy desert. Halley's Comet revealed long canyons, wide craters and up to 900-meter high hills. Nobody knows whether Wirtanen might look similar.

The German camera will also look for level terrain, since Rosetta will deploy a lander on the surface some time after having reached the comet.

Originally, two landers had been planned. One, called Champollion, was to be jointly built by France and the USA, the other one, Roland (Rosetta Lander), was planned as an exclusive German project. After the Americans' decision to withdraw from the project, Germany and France will jointly build one lander. This vehicle will land softly on the surface and will then have to be anchored. The reason for this is that the comet, measuring only a few kilometers, has an extremely low gravity. Its force of attraction is so low that a coin thrown in the air would disappear forever.

The lander will carry a series of complex instruments for the analysis of surface samples. Additionally, a camera will take panoramic pictures. The camera is a cooperative development between the Institute for Planetary Exploration of the German Aerospace Center (DLR) in Berlin and French scientists. It is even planned to lower a micro-camera into the bore-hole to determine the structure of the cometary crust.

While Rosetta orbits the comet, the latter will steadily approach the Sun. Wirtanen will wake up from the icy cold and start to heat up. Gases will evaporate from its surface and inside. They will drag along dust particles, and Wirtanen will thus form a coma and a tail. The 13 measuring instruments on board Rosetta will then begin the study of gas and dust. Approximately one year after Rosetta's encounter with the comet, Wirtanen will have reached the orbital point nearest the Sun, where it is most active. Shortly thereafter, the spacecraft mission will be completed. Wirtanen will then withdraw again into the outer and icy regions of the planetary system.

For the first time, scientists will thus have the opportunity to see "live" how a cometary surface evolves in the course of the "seasons". The pictures taken by Giotto already showed that, apparently, huge gas jets spout out of the crevasses in the surface. Rosetta will show this process in much more detail and thus help unravel further mysteries that surround the comets.

Comets - the Archives of Primeval Times
The gathered data will be of inestimable value to the scientific community, as the instruments onboard the lander, in a way, will tap an archive which has preserved unaltered material from the time of formation of the solar system. Pristine matter can be found on no other celestial body, with the possible exception of some asteroids. On the Earth and also on other planets, source material from which the planets were formed, continuously evolved in the course of the last 4.6 billion years. Chemical, geological and biological processes transformed the atmosphere and rocks thereby destroying any information from primeval times.

Comets, by comparison, are so small that no geological processes such as erosion or plate tectonics take place. There are presumably billions of cometary nuclei that slowly move around the Sun far outside Pluto's orbit. It is only when one of them ventures into the inner solar system that it will heat up so that gas will evaporate and it will appear as a tailed star in the sky.

Comets spend most of their "life" in the outer regions of the solar system where temperatures drop to nearly absolute zero. The cometary material is thus preserved in a deep-freeze state. Scientists therefore hope that Rosetta will help them determine the chemical composition of protosolar nebula in order to further explore the creation OT our solar system and thus of the Earth. In other words: Rosetta will be looking for the roots of our existence.

This also explains the name of the spacecraft. It refers to the "Rosetta Stone", an Egyptian inscription stone, discovered by a soldier in the Napoleonic Army in 1799, near the Egyptian city Rosetta. By means of the inscription on that stone, the French scholar, Jean-Francois Champollion, detected the first clues to deciphering the Egyptian hieroglyphics, thereby opening the historians' way to our cultural roots.

Hubble poster
The Hubble Space Telescope's majestic view of the Eskimo Nebula. This spectacular poster is available now from the Astronomy Now Store.